Developing or donor rollers for printers are well known, these rollers often being electrostatically charged to carry charged ink toner from a toner supply to a print cylinder. A latent electrostatic image carried on the print cylinder then attracts the toner from the donor roll onto the print cylinder. The image is thereby developed on the print cylinder.
For example, U.S. Pat. No. 5,315,061 to Suzuki et al. shows a developing roller having a conductive surface with dielectric bodies thereon. A charged toner is attracted to the roller by micro fields which arise between the dielectric bodies and the conductive surface. The toner then is transferred to a print belt which carries a latent image to be developed. A doctor blade can limit the thickness of the ink carried by the developing roller.
In prior art systems using a developing roller, such as the '061 patent, the developing roller only contacts the print member surface in a single location.
However, proper development of an image requires a certain minimum time for the latent image on the print surface to attract the necessary ink or toner. By contacting only a single location of the print surface, the developing rollers of the prior art are limited by the time necessary to develop properly the image on the print surface.
The developing roll prior art devices thus limit the speed and quality of the developed image, since the quality of the developed image, the narrowness and repeatability of its various tolerances, the insensitivity to ambient conditions during development, and the brevity of time or "speed" with which development can be accomplished depend largely on the nature of the ink applicator and its interaction with the print member.
U.S. Pat. No. 3,689,933 shows a developing station which covers a larger area of a print cylinder surface than a developing roller. However, the specifics of the developing station, including its reliability and quality, are not described.
European Patent Application 0141 663 shows a donor roller contacting a print surface at a single tangential point of a print cylinder. It also describes that an endless belt may be substituted for the donor roller, although this belt as well presumably would only contact the print cylinder at one point.